Seafloor Sensors Listen to Quake Zone Rumblings

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The devastating might of Earth's mysterious movements was put on
tragic display this year when the Tohoku earthquake struck off
the coast of Japan.

The
magnitude 9.0 quake was produced by a sudden lurch along a
subduction zone, a place where one tectonic plate dives beneath
another in an epic, slow-motion collision.

Unbeknown to many on the other side of the Pacific Ocean, a
similar subduction zone lurks along the North American coastline.

The Cascadia subduction zone — a tectonic border region where the
oceanic Juan de Fuca plate is grinding slowly beneath the North
American plate — stretches from Northern California up to
Vancouver Island. And although scientists are busily studying the
fault, which has produced
massive earthquakes in the past, some aspects of Cascadia's
character and structure remain largely mysterious.

"We don't really know very much about what's going on," said
Andrew Barclay, a research scientist at Columbia University's
Lamont-Doherty Earth Observatory.

Seafloor sensing

Barclay and a group of colleagues recently returned from an
expedition to install 15 newly developed sensors off the coast of
northern Washington state, in the first U.S. attempt to gather
data on the portion of the fault trickiest to study — the
part that lies under the ocean.

In late July, researchers spent more than a week aboard a
research vessel, hoisting the 1,500-pound (680 kilograms)
instruments overboard and, with the help of very long cables and
a sturdy crane, settling each massive sensor on the seafloor.

The first instrument stands about 15 miles (24 kilometers)
offshore. The devices, armed with seismometers and other sensing
equipment, are spaced more than a mile apart in a line that
marches out to sea. They will serve as silent sentinels, taking a
torrent of data on the Earth's movements — whether minute or
massive — at a rate of 125 samples per second. [Related: 7 Ways
the Earth Changes in the Blink of an Eye ]

The instruments, which resemble stooped pyramids, will not give
up their secrets until a year from now, when scientists return to
wrestle the devices off the seafloor, crack them open and see
what data are captured inside.

"We expect to be able to hear magnitude 6.5 earthquakes — and
above — from around the world, as well as local earthquakes,"
Barclay told OurAmazingPlanet.

One big incentive for deploying the instruments is that they will
provide a way to
image the Earth's insides, using the waves of energy sent
from faraway earthquakes to make a geological map of the rocks
that comprise the Cascadia fault. Knowing the arrangement of
rocks, sediment and liquid inside the subduction zone will help
scientists better understand how it works.

"It's somewhat analogous to doing a combination CAT scan and
ultrasound," said Anne Trehu, a professor at Oregon State
University and co-head of the recent expedition.

Yet it is the data on Cascadia's movements — data scientists
can't get without putting a technological ear right against the
subduction zone — that may help solve one of the fault's most
enduring mysteries: Is the fault primed for a big earthquake?

Violent history

At first glance, the Cascadia fault appears to be a quiet giant.
It has produced only a few earthquakes in recent decades. The
largest were California's magnitude 7.2 Petrolia earthquake in
1992, and Washington's magnitude 6.8 Nisqually earthquake in
2001.

Along the plate boundary, on the fault's seaward side, the quakes
have fallen in the magnitude 4 range, none of them damaging. "It
has been fairly quiet in our memories," Trehu said.

Yet scientists aren't fooled by Cascadia's relative timidity in
recent years. The geologic record indicates the subduction zone
has made dramatic moves in the past.

"You can draw analogies with earthquakes in recent years and the
tsunamis from them," said Brian Atwater, a geologist with the
U.S. Geological Survey who has studied the telltale signs left
behind by Cascadia's last sudden rupture.

Atwater pointed to the earthquakes and ensuing tsunamis that
shook Chile last year and later devastated Japan.

Cascadia has
produced megathrust quakes of a similar scale for thousands
of years, but "centuries pass between successive ones," Atwater
said, "and that's why we don't have one within our 200 years of
written history."

Secret earthquakes

Although Cascadia has kept largely silent for 300 years,
scientists recently made a surprising discovery: Deep inside the
fault, beyond the zone that produces bone-shaking earthquakes
people can feel, Cascadia keeps up a near-constant murmur.

"We know now that it is slipping fairly regularly deep down, and
it's not slipping shallower, where it's locked," Trehu said.

About a decade ago, researchers discovered that between 22 and 34
miles (35 to 55 km) below the surface of the Earth, Cascadia
produces strange, slow-motion
earthquakes that creep along the fault at about 4 mph (6.4
kph), for two weeks at a time.

In contrast, damaging earthquakes that rupture closer to the
surface streak along a fault at a supersonic clip — about 2 miles
(3.2 km) per second.

Though they are imperceptible except to the most finely tuned
instruments, the slow-motion earthquakes pack a seismic punch,
all falling in the magnitude 6 range, in terms of the total
amount of energy they release.

They also occur with astonishing regularity.

Known as Episodic Tremor and Slip (ETS) in scientific parlance,
the earthquakes amble along the Cascadia fault roughly every 14
months in near-predictable cycles — and in maddening contrast to
their destructive counterparts, which are notoriously difficult
to forecast.

Many scientists think the slow quakes deep inside the fault could
be building up stress in Cascadia's shallower regions,
essentially prepping the fault for a large earthquake. However,
the location of the slow earthquakes is important to this theory.
"It depends on where it's occurring," Trehu said.

Answering that question will send scientists well on their way
toward solving what amounts to a high-stakes seismic algebra
problem.

Solve for x

On one side of the equation is the fault's overall movement over
time. Scientists have a basic understanding of that figure. "We
know what the large-scale motion should be," Trehu said.

To complete the equation, scientists have to hunt down all the
sources of that movement. Regular earthquakes are one source, and
a source that scientists can measure with some degree of
precision — even the tiniest earthquakes broadcast their
activities loudly enough for instruments to hear them.

However, slow-motion earthquakes — ETS — are sneaky. Instruments
must be close by to pick up on them.

The newly installed ocean seismometers are designed to listen for
slow-motion quakes along the seaward, shallower regions of the
fault. If they are indeed occurring there, it could be that the
fault isn't primed for disaster — at least not now.

"If the whole fault is slipping slowly, then it's not locked,"
and thus not storing up energy for a large earthquake, Trehu
said.

Barclay said he's looking forward to getting his hands on the
data a year from now, hoping they'll indicate whether the
slow-motion earthquakes are occurring. "It's exciting for me
because it's episodic — and up to now our idea of subduction
zones is that things happen at random," he said.

However, Trehu said, the project has even grander ambitions than
sniffing out hard-to-detect quakes and imaging the subduction
zone. Ultimately, the goal is to understand how the entire system
works —
how and why earthquakes happen at all.

"It's poorly understood what factors control whether the rupture
continues to propagate, what factors stop the rupture," she said,
"and it's those factors that determine how big the earthquake
is."

Scientists from more than six universities and institutions
around the country have banded together for the Cascadia project,
a years-long undertaking that is funded, in part, by the American
Recovery and Reinvestment Act.

The researchers will make the data freely available to anyone who
wishes to use it.

In the meantime, Trehu and Atwater said, people who live within
the reach of the Cascadia subduction zone should be aware of the
fault's potential. [Related: 7 Most Dangerous
Places on Earth ]

"People should definitely think about the fact that a large
earthquake is probable, eventually," Trehu said. "A tsunami
that's generated in Cascadia will only take tens of minutes to
get to the coast. So if you're at the beach and you feel an
earthquake, the best thing to do is get to higher ground as fast
as you can."